The number of nozzles of a printhead used in an inkjet printer increases year by year. Along with this, the manufacture of the printhead becomes more difficult year by year. The manufacturing yield of the printhead must be increased by decreasing the number of defective printheads having nozzles which fail to discharge ink (to be referred to as discharge failure nozzles hereinafter).
As a method of increasing the manufacturing yield of the printhead, there has conventionally been known complement of discharge failure. According to this technique, printing data which cannot be used for printing owing to a discharge failure nozzle is printed using another nozzle, thereby compensating dots which cannot be printed. This technique is disclosed in, e.g., Japanese Patent Publication Laid Open Nos. 8-025700 and 11-000988.
By using complement of discharge failure, a printhead which is screened as a defective printhead can be employed as a nondefective printhead, and the yield of the printhead can be increased.
In conventional complement of discharge failure, complementary printing is performed using several nozzles (to be referred to as target nozzles hereinafter) arranged on the two sides of a discharge failure nozzle in the nozzle array of a printhead. More specifically, in complementary printing, nozzles near a discharge failure nozzle discharge ink by using printing data which should be originally printed by the discharge failure nozzle. The printing duty is controlled to make less conspicuous a white stripe which appears on a printing medium owing to an ink discharge failure.
The inkjet printer conventionally has a nozzle restriction function. The nozzle restriction function restricts the number of available nozzles of the printhead to reduce the printing data amount. As a result, the capacity of a memory used to store printing data, and the data transfer amount from the memory to the printhead can advantageously be reduced.
In the use of both the discharge failure complement function and nozzle restriction function, however, if a discharge failure nozzle is created near the boundary between an area where the use of the nozzles of the nozzle array of the printhead are restricted and an area where the nozzles are free from any restriction, a target nozzle used for complement of discharge failure may fall within the area where the use of the nozzles are restricted. In this case, complementary printing for discharge failure by using a target nozzle cannot help printing in an area where printing should not be originally done. As a result, the quality of a printed image degrades.